The hedge fund said it remained "extremely interested" in buying the networking equipment maker. But it stuck to the $3.36 billion offer it made in February, and that Riverbed rejected as too low last month.

Elliott said it would consider another increase in the $21 per share offer, but only if it is given access to Riverbed's books. In March, Elliott criticized the company for refusing to allow potential buyers to do due diligence.

"Shareholders clearly still expect that the board will eventually do the right thing by engaging with Elliott and other potential buyers, but unfortunately our repeated requests for access to diligence materials have been ignored," Elliott's Jesse Cohn said.